(CNN)The jailed vice president of Rwanda's opposition FDU-Inkingi party escaped from prison on Sunday, according to the country's correctional service.

Boniface Twagirimana was missing from a routine headcount at the prison Monday, local media reported, quoting a Rwanda Correctional Service spokesperson. The spokesperson said that Twagirimana and another prisoner had managed to escape by jumping over the complex's fence and said that an investigation had been launched.

But members of the FDU -- an unregistered political party -- are calling "foul play" and fear that Twagirimana's life could be in danger.

In a statement released Monday, the FDU party questioned how Twagirimana could have escaped out of a high security prison he had been transferred to only five days prior and called on the Rwandan government for answers.

"This information...leaves us to believe that there could be foul play by Rwandan security services," the statement said.

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"We call on the Rwandan government to inform the family, the party FDU-Inkingi and the general prison about the circumstances of the disappearance of Twagirimana. Mr Twagirimana was in the custody of the state which is accountable for his safety," it added.

In September 2017, Twagirimana and eight other FDU party members were arrested on charges of forming an armed group and seeking to overthrow the government, charges Twagirimana denies.

The FDU members were placed in a Kigali jail where their party leader, Victoire Ingabire, was serving out a sentence for charges related to comments she made about the country's 1994 genocide and collaborating with a "terrorist organization."

Ingabire has long said her sentence was a result of her work as a prominent government critic and that the charges effectively criminalized her freedom of expression. International organizations such as Amnesty International and a 2017 African Court on Human and Peoples' Rights ruling have supported those views.

Last month, Ingabire was granted a presidential pardon by Rwanda's President Paul Kagame and was released from jail after serving eight years of her 15-year sentence.

Immediately after she was freed, she called on the Rwandan government to open the country's political landscape to the opposition and asked them to free all other political prisoners, including Twagirimana and other members of her political party.

On October 3, Twagirimana was moved from Kigali's Mageragere prison to Mpanga prison, in the country's southern Nyanza District. The authorities did not inform Twagirimana's family that he was being transferred or give any explanation for the move, according to Twagirimana's wife.

Rwanda's National Police and Rwanda's Correctional Service have not immediately responded to CNN's request for comment.

Twagirimana is not the first FDU member to go missing.

In May 2017, party member Jean Damascene Habarugira disappeared after he was called to meet an official responsible for the security of his locality. A few days later, Habarugira's family were called to collect his body from a local hospital.

Twagirimana denounced Habarugira's murder as an assassination. In a statement, the FDU said that Habarugira was "assassinated in cold blood" because of his opposition to the local authority's agricultural policies and concerns over police brutality.